The only time the TankItem is replot is when new data is set.
Therefore, replot() can be folded into setData().
The good thing is that setData() is passed the dive to be
plot. So the data can be extracted from there instead of
the global displayed_dive variable.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
0146a0c892